The starting of air breathing, gas turbine engines at high altitudes presents substantial difficulties, particularly in the case of relatively small gas turbine engines. At high altitudes, the temperature of the environment is quite cold with the consequence that fuels have high viscosity making it quite difficult to atomize the fuel sufficiently to ignite properly.
Furthermore, and as is well known, in the operation of turbine engines, the higher the altitude, the lower the fuel flow required to maintain any given standard of operation. Consequently, at high altitudes, relatively low fuel flows are required and that in turn means a reduction in the pressure applied to the fuel to achieve the reduced flow rate. Thus, where the turbine fuel injectors are of the pressure atomization type, the lesser fuel pressure utilized at high altitude means insufficient pressure to cause the required degree of atomization necessary to achieve a start. If it is attempted to overcome this difficulty by increasing the pressure, frequently, expensive altitude compensation control systems for fuel flow must be added to the system and even then, there will frequently be over fueling of the engine which in turn results in hot spots once ignition is obtained.
Further, particularly in the case of relatively small turbine engines, it is necessary to utilize so-called "start injectors" in addition to main fuel injectors. Start injectors are specially designed to provide the desired degree of atomization at maximum operating altitudes and typically are used only during the starting operation. They are turned off after ignition is obtained with fuel thereafter being supplied by the main fuel injectors. Again, the use of special injectors such as start injectors undesirably adds to the cost of the engine and still may not provide the desired degree of start reliability at high altitude of, say, thirty thousand or forty thousand feet.
The present invention is directed to overcoming one or more of the above problems